Luke - could you try bypassing the permission_p procedure and do the query directly from psql?
select count(*)
from acs_object_grantee_priv_map
where object_id = 2920
and grantee_id = 2082
and privilege = 'bboard_create_forum';
It's possible that there is a problem with the acs_object_grantee_priv_map view.